Nearly 400 years after Galileo first peered at the heavens through a rudimentary telescope, the world's top astronomers have called an urgent meeting to decide once and for all the meaning of the word "planet".

The answer, which will be mulled over by committee before being put to the vote at the International Astronomical Union meeting in Prague today, will end what has become an embarrassing crisis for scientists. It will also change the nature of the solar system forever: where we now have nine planets endlessly looping around the sun, we may soon have only eight. Or 23, or 39, or more.

At the root of the crisis is a 76-year-old celestial fudge that has until now been papered over: the discovery of Pluto. When scientists at the Lowell Observatory announced they had spotted Pluto in 1930, they claimed it was several times larger than Earth, ensuring its prompt entry into the textbooks as the ninth planet. It later turned out to be a rock substantially smaller than the moon. "Their public relations was great, but their astronomy was lacking a bit," said Brian Marsden, director of the IAU's Minor Planet Centre in Cambridge, Massachusetts.

Until last year, the astronomy community was mostly happy to turn a blind eye to Pluto's apparent crashing of the planetary party. But last year, Mike Brown, an astronomer at California Institute of Technology, discovered a fly in the ointment -another celestial body larger than Pluto, hurtling around with an orbit stretching beyond Neptune.

The Hubble space telescope measured the rocky object at about 1,490 miles in diameter - roughly 70 miles longer than Pluto's. While it is officially known as 2003 UB313, Professor Brown named the rock Xena, after the Warrior Princess TV series, and claimed it as the 10th planet.

"The discovery of this object really brought things to a head. It's spherical, it orbits the sun, and it happens to be bigger than Pluto," said Dr Marsden.

The reason the question has become such a headache is that unlike the inner eight planets orbiting the sun, Pluto is a member of a vast band of extremely distant rocks called the Kuiper belt that orbits the sun beyond Neptune. Xena is a Kuiper belt object too, so if Pluto qualifies as a planet, so must Xena.

And the headache does not end there. Astronomers know that the Kuiper belt is likely to be full of countless other rocks, so where do they draw the line?

"It's time we have a definition," said Alan Stern, who heads the Colorado-based space science division of the Southwest Research Institute of San Antonio. "It's embarrassing to the public that we as astronomers don't have one."

"Life would be so much easier if we just had eight planets and a lot of very interesting small things in the solar system," said Dr Marsden. "Don't get me wrong, I'm not anti-Pluto, it's a very interesting object, but so are a lot of others out there. The thing is a lot of people are very possessive about Pluto."

Astronomer Patrick Moore is not one of the sentimentalists. "I hate to say it, but if Pluto's a planet, there are plenty of others out there that should also be planets. But a planet it isn't. Unfortunately, I think we only have eight planets in the solar system, not nine."

Iwan Williams, president of the IAU's planetary systems sciences committee and astronomer at Queen Mary University, London, said: "Personally, I'd include them all as planets, but in normal conversation, we'd slightly qualify all our planets and say we have four terrestrial planets, four giant planets and however many dwarf planets."

A popular idea is to count only objects above a certain size. Some scientists want an arbitrary limit - a diameter of around 1,243 miles. Others say the size and rotation of the object should be sufficient to give the rock a strong enough gravitational field to make it spherical.

Either way, our nearest celestial neighbours could then be categorised as Prof Williams suggests: four terrestrial, or rocky, planets - Earth, Mercury, Venus and Mars; four giant planets - Jupiter, Saturn, Uranus and Neptune, and a number of dwarf planets, such as Pluto. Depending on what the IAU decides, humanity could wake up next week in a solar system with as many as 50 new planets. "Pluto is not worthy of being called just a plain planet," said Alan Boss, an astrophysicist at the Carnegie Institution in Washington. "But it's perfectly fine as an ice dwarf planet or a historical planet."

Prof Brown yesterday told the Associated Press that he could accept eight planets, but would feel guilty if Xena gained planethood because of the controversy surrounding Pluto. "If UB313 is declared to be the 10th planet, I will always feel like it was a little bit of a fraud," he said.

Whatever the outcome, the IAU's decision could rewrite school textbooks and encyclopaedias and countless science fiction stories. "I don't know about the public, but whatever happens, the astrologers will be upset about it," said Mr Moore.

FAQ 'Wanderers'

What makes a planet a planet?

There are no hard and fast rules. The term comes from the ancient Greek for "wanderer" because the first astronomers noticed that they moved against the background of the stationary stars in the sky. Today, a planet is thought of loosely as a non-luminous object orbiting a star. Unfortunately, that definition includes thousands of comets and asteroids.

What's the issue with Pluto?

For a start, it is not very big - the moon is 1.5 times bigger. Bigger objects have been found in the solar system in recent years including Quaoar (2002) and Sedna (2004) and neither attained planet status. Pluto is ripe for redefinition but some astronomers feel that demoting the ninth planet would be an unpopular decision.

What are the possible definitions?

Many astronomers suggest setting a size limit: anything under 2,000km (1,243 miles) wide should not be a planet. Others want more constraints: a planet should be spherical, should be dominant in its immediate neighbourhood or should contain more mass than anything else in the same orbit around the sun.

Why are astronomers deciding now?

The situation came to a head last year with the discovery of a 1,860-mile-wide object called 2003 UB313, nicknamed Xena. It is the biggest object found in the solar system since Neptune in 1846. If Pluto counts as a planet, so should Xena. The International Astronomical Union, which decides the names of celestial objects, subsequently agreed to come up with a scientific definition for planets. The most likely outcome seems to be one where Pluto keeps its status, probably for sentimental reasons, and Xena will be officially confirmed as the solar system's 10th planet.